Textstats package

Install Package. Hit cmd+shift+t and toggle a window with some statistics about what you've written. Includes Word Count, and Flesch Kincaid Reading Ease and Grade Level scores.

I admittedly am not a full time programmer, and really have no idea about Atom's package framework. It's an attempt to emulate writeforhumans.io -- I've not yet managed to learn how to get into the status bar yet. But this is a helpful start.